The use of microencapsulated ingredients in the manufacture of pharmaceuticals, pesticides, paints, and water-based adhesives is known. The most widely-known use of microcapsules has been in the product generally known as carbonless paper wherein microcapsules provide a controlled release of a color forming reagent core which is released when pressure is applied.
Landis, U.S. Pat. No. 5,532,293 discloses an adhesive system for tamper-evident envelopes. The adhesive system comprises a dried cohesive latex containing gelatin and a plurality of solvent-filled microcapsules. Applying pressure ruptures the microcapsules causing the solvent to be released, dissolving the cohesive material and forming a bond. The solvent dissolves the cohesive material without substantially dissolving the non-adherent material.
Japanese Patent Application No. 6 [1994]-172725 discloses a water dispersible resin composition containing 10.0 wt-% to about 90.0 wt-% of encapsulated plasticizer alone or in combination with a tackiness-rendering agent having non-adhesive shells and having an average particle diameter of 10-300 .mu.m and 10.0 wt-% to about 90.0 wt-% of a tackifier characterized by increased tackiness after the shells of the encapsulated plasticizer are destroyed with pressure. This patent application teaches the use of conventional water based plasticizers such as phthalic acid esters and phosphoric acid esters in combination with conventional water-based tackifiers such as acrylic-type tackifiers, natural rubber, butyl rubber, styrene-butadiene rubber and other synthetic rubber compounds.
U.S. Pat. No. 2,800,458, issued Jul. 23, 1957 teaches oil-containing microscopic capsules and the method of making, for use in aqueous dispersion transfer films.
Although water-based adhesive compositions comprising microencapsulated solvent and plasticizer have been disclosed to some extent as mentioned above, microencapsulated ingredients have never been utilized in a hot melt adhesive composition.
EP 0 531 618 A1 teaches a cohesive composition, which is tack free when cooled to room temperature and will only adhere to itself, including 60 to about 95 parts by weight of an amorphous polyalphaolefin hydrocarbon. This composition is useful as a cold seal composition for packaging, and related applications. Cohesive cold seal compositions are those which exclusively form bonds to themselves at room temperature.
Commercially available water-based glue-sticks typically are supplied in a container for the purpose of protecting the adhesive from prolonged exposure to air, which will solidify the glue-stick surface, as well as to support and contain the adhesive, as such compositions are much too pliable to maintain a stick like shape in the absence of a container.
Hot melt adhesives are used widely for a variety of applications including packaging and bookbinding wherein various wood pulp based substrates are bonded to each other. Hot melt adhesives typically comprise at least one thermoplastic polymer and at least one additional thermoplastic ingredient selected from the group consisting of tackifying resins, plasticizers, waxes, and mixtures thereof. Additionally, for hot melt pressure sensitive adhesives (PSAs) at least one ingredient is normally a liquid at ambient temperature. The liquid ingredient imparts the pressure sensitivity or surface tackiness at ambient temperature. Hot melt PSAs are most commonly applied to tapes, labels, medical devices and for various uses in disposable products such as disposable diapers and feminine napkins, for product assembly, elastic attachment, and garment attachment applications. Hot melt PSAs are often the preferred adhesive technology due to the ability to bond difficult to bond substrates such as polyolefin films and nonwoven materials at high line speeds without the additional drying step that is needed for water-based and solvent-based adhesives. The introduction of water or other solvents is particularly a problem for absorbent products as it is very difficult (if not impossible) to completely dry the absorbent. Hot melt PSAs also provide hot tack and immediate green strength, allowing for very high manufacturing rates.